Ultramafic Blocks in Sumdo Region, Lhasa Block, Eastern Tibet Plateau: An Ophiolite Unit

Recently, an over 100 km long MORB-type eclogite belt of Permian was discovered in the Sumdo ((sic)) region of the Lhasa block, Tibet. A critical question thus is: what is the tectonic setting of the eclogite belt and is it related to an unrecognized suture in the region? Further investigations show that there are some mafic and ultramafic rocks spacially associated with the eclogite belt in the region. Three ultramafic massifs were recognized in the Sumdo region, and called the Luomaling ((sic)) Gongbupala ((sic)) and Qiazhasumdo ((sic)) massifs. All the massifs are fault-contacted with greenschist (Chasagang ((sic)) Formation) or muscovite-quartz schist (Mabuku ((sic)) Formation), and individuals are about 100 mx50 m in size extending in EW as the regional structure. All the ultramafic rocks have been entirely serpentinized, and the Gongbupala massif has been selected for study in geochemistry. Eleven chemical analyses of the rocks from the Gongbupala massif show a narrow range in contents: SiO2 (35.97-40.63) wt.%, MgO (37.02-38.60) wt.%, TiO2 (0.01-0.08) wt.%, Al2O3 (0.80-1.64) wt.%, (Na2O+K2O) less than 0.1 wt.%, with high volatile contents (H2O+CO2) (11.24-14.91) wt.%. After recalculation without H2O+CO2, the mean values are SiO2 45.24 wt.%, MgO 43.54 wt.%, FeOT (7.45-9.97) wt.% (8.55 wt.% in average), (MgO+FeOT,) 52.09 wt.%, Mg-# (100xMg/(Mg+Fe*), where Fe* represents total Fe)=89.42-90.08, (m+f)/Si ((atomicity Mg+atomicity Fe) /atomicity Si)=1.53-1.75 (1.59 in average), respectively. The mean M/F (atomicity Mg/atomicity Fe) ratio of the rocks is 9.05, which is classified as magnesium enriched-type of ultramafic rocks. The compositional features, depleted in K, Na, Ca, Al and Ti and enriched in Mg-#, indicate the characteristics of peridotite originated from a depleted mantle. The rocks have low Sigma REE with (1.60-2.68)x10(-6) similar to those of the primitive mantle. The chondrite-normalized REE patterns of all samples show slightly enrichment in LREE, with (Ce/Yb)(N) 1.03-2.46, but a little depleted in HREE. Most samples show a slight negative anomaly in Eu, a feature in REE from a relic mantle and common features in highly serpentinized ultramafic rocks in the Yarlung-Zangbo ((sic)) ophiolite and the Bangong ((sic))-Nujiang ((sic)) ophiolite in Tibet. The primitive mantle-normalized spiderdiagram of trace elements for Gongbupala ultramafic rocks yields uniform distributed pattern. They are relatively enriched in Rb, Ba, La, P element (LHSE) and depleted in Sm, Ti, Y, Yb element (HFSE), a feature of metasomatic mantle peridotite. The geochemical features of the rocks suggest that the protolith of Gongbupala serpentinite in Sumdo region is harzburgite, a typical depleted mantle rock, and may represent a dismembered ophiolite unit in the region.